Method for improving the performance of nickel electrodes

ABSTRACT

The invention relates to a method for improving the performance of coated nickel electrodes in chloralkali electrolysis by adding water-soluble platinum compounds to the catholyte during the electrolysis at low current density.

The invention relates to a method for improving the performance of nickel electrodes, in particular of noble metal-coated nickel electrodes for use in sodium chloride electrolysis.

The invention proceeds from the known use of nickel electrodes as hydrogen-evolving electrodes in alkali metal chloride electrolysis and the improvement methods known per se by coating nickel electrodes with noble metals or noble metal oxides.

Cathodes for sodium chloride electrolysis, at which hydrogen is evolved in alkaline solution, usually consist of iron or nickel. If nickel electrodes are used, these can consist entirely of nickel or only nickel surfaces in which substrates composed of other metals are nickel plated on the surface are used.

As stated in EP 298 055 A1, nickel electrodes can be coated with a metal of transition group VIII, especially the platinum metals (including Pt, Ru, Rh, Os, Ir, Pd), of the Periodic Table of the Elements or an oxide of such a metal or mixtures thereof.

The electrode produced in this way can, for example, be used in sodium chloride electrolysis as cathode for the evolution of hydrogen. Here, many coating variants are known since the coating composed of metal oxides can be modified in various ways so that different compositions are formed on the surface of the nickel electrode. In U.S. Pat. No. 5,035,789, a coating based on, for example, ruthenium oxide on nickel substrates is used as cathode.

During operation of the nickel-based electrodes, a decrease over time in the quality of the electrode is observed, so that the cell voltage in the sodium chloride electrolysis increases, which may make fresh coating of the electrode necessary. This is technically complicated since the electrolyzers have to be switched off and the electrodes have to be removed from the electrolysis cells.

It is therefore an object of the invention to find a simpler form of improving the performance or restoring the performance which does not require dismantling of the electrolysis cells.

According to the teaching of U.S. Pat. No. 4,555,317, iron compounds or finely divided iron are added to the catholyte in order to decrease the cell voltage in sodium chloride electrolysis. In the case of noble metal-coated nickel electrodes, covering the cathodes with iron can have an adverse effect on the electrolysis and increase the cell voltage.

In the further published document EP 1 487 747 A1, a compound containing from 0.1 to 10% by weight of platinum is added to the sodium chloride electrolysis. Here, the solution of the platinum-containing compound is introduced into the water which forms the catholyte, with from 0.1 to 2 liters of the aqueous solution containing the platinum compound being added per liter of water. EP 1 487 747 A1 discloses no information about the conditions, electrodes, electrode areas, current density, etc., used in the process, as is required for industrial implementation, apart from the general reference to the use of the platinum compound during the electrolysis.

In JP 1011988 A, a soluble compound of a metal of the platinum group is added to the sodium hydroxide solution in the catholyte during operation of the sodium chloride electrolysis to restore the activity of a deactivated cathode based on a Raney nickel structure having a low hydrogen overvoltage. For example, a sodium chloride electrolysis cell is operated with 32% strength by weight sodium hydroxide, a salt concentration of 200 g/l of sodium chloride at 90° C. and a current density of 2.35 kA/m². For pretreatment, the cathode is electrolessly coated with nickel and subsequently nickel plated in a nickel bath. As active compound, platinum chlorate, for example, was added to the catholyte, which led to a decrease in the cell voltage by 100 mV.

According to U.S. Pat. No. 4,105,516, metal compounds are added to the catholyte during the electrolysis of alkali metal chlorides; these are said to decrease the hydrogen overvoltage and thus reduce the cell voltage. The examples set forth in U.S. Pat. No. 4,105,516 again describe the added amounts and effects arising from addition of an iron compound which is added to the catholyte of a laboratory sodium chloride diaphragm cell. The cell has an anode which consists of titanium expanded metal coated with ruthenium oxide and titanium oxide. The cathode here consists of iron in the form of expanded metal. The examples disclose the use of cobalt or iron solution at the iron cathode. The disadvantages of iron compounds in the treatment of coated nickel electrode have been indicated above.

It is known from the further known patent document U.S. Pat. No. 4,555,317 that the sodium chloride electrolysis can be started using a nickel-coated copper cathode. Here, an active coating composed of nickel and cobalt dendrites is applied, with a porous platinum layer having been deposited electrolytically beforehand as intermediate layer for anchoring the dendrites. An initial addition of hexachloroplatinic acid to the cell under electrolysis conditions was carried out in a plurality of steps at a high current density of 3 kA/m².

According to U.S. Pat. No. 4,160,704, metal ions which have a low hydrogen overvoltage can be added to catholytes of a membrane electrolysis cell for sodium chloride electrolysis in order to coat the cathode. Here, the addition is carried out during the electrolysis. However, only the addition of platinum oxide for improving an iron or copper cathode is indicated by way of example.

The cathode coatings in sodium chloride electrolysis usually consist of platinum metals, platinum metal oxides or mixtures thereof, e.g. a ruthenium/ruthenium oxide mixture.

As described in EP 129 374, the usable platinum metals include ruthenium, iridium, platinum, palladium and rhodium. The cathode coating is not stable in the long term, especially not under conditions at which no electrolysis takes place or during interruptions to the electrolysis during which electric reverse currents, for example, can occur. Thus, more or less severe damage to the noble metal coating occurs over the time of operation of the electrolyzer. Likewise, impurities which, for example, get from the brine into the alkali by diffusion, e.g. iron ions, can deposit on the cathode or especially on the active sites of the noble metal-containing coating and thereby deactivate the latter. The consequence is that the cell voltage increases further during operation of the electrolysis, as a result of which the energy consumption for the production of chlorine, hydrogen and sodium hydroxide is increased and the economics of the electrolysis process are significantly impaired. Likewise, only individual electrolysis cell elements of an electrolyzer can have damage to the cathode coatings. Here, it is technically highly complicated and therefore uneconomical to switch off the entire electrolyzer for replacement of the damaged cell elements and to remove the cell elements having the damaged coating, since this is associated with production downtimes and considerably costs.

Still a further method for improving noble metal-coated nickel electrodes has become known from DE 102007003554A1, in which a hexachloroplatinate solution or sodium hexachloroplatinate solution is metered into the sodium hydroxide solution in which a cathode coated with ruthenium oxide is operated during operation of the sodium chloride electrolysis at a production current density in the region of several kA/m². Here, a variation of the cell voltage in a voltage range from 0 to 5 volt or 0.5-500 mV and a frequency of 10-100 Hz with an amplitude of 20-100 mV is said to be carried out. The introduction of the platinum compound into the catholyte is carried out, in particular, into the feed stream to the cathode chamber at a cathode area of 2.7 m² and a current density of from 1 to 8 kA/m². The metering rate of the platinum-containing solution based on the platinum content per m² of cathode area is in the range from 0.001 g of Pt/(h*m²) to 1 g of Pt/(h*m²).

A disadvantage of the coating method disclosed in DE 102007003554A1 is that the positive effect which is initially achieved by the platinum doping described cannot be maintained during a stoppage of the electrolysis. Since electrolyzers go down or have to be switched off for various technical reasons, a renewed introduction of platinum has to be carried out after each downtime, as a result of which additional complexity is introduced into the operating procedure, representing a disadvantage for production operation. The coating of the electrode with platinum or platinum oxide which can be achieved by this method is obviously not as stable as would be desirable for the production process. In addition, platinum may sometimes not be deposited completely from the platinum solution onto the electrode surface and rare and costly noble metal material is thus lost.

It is an object of the invention to develop a specific method for improving nickel electrodes which have been coated with platinum metals, platinum metal oxides or mixtures thereof or for nickel electrodes without coating for use as cathodes in the electrolysis of sodium chloride, which method can be used in ongoing electrolysis operation, avoids a relatively long interruption of electrode operation for restoring the cathode activity and gives an improvement in the activity of the nickel electrodes which is not lost immediately in the event of a stoppage. In particular, the method should not impair the function of the operating plant for the electrolysis.

The invention provides a method for improving the performance of nickel electrodes which are uncoated or have a coating based on platinum metals, platinum metal oxides or a mixture of platinum metals and platinum metal oxides and are used in sodium chloride electrolysis by the membrane process, where a platinum compound which is water-soluble or soluble in sodium hydroxide solution, in particular hexachloroplatinic acid or particularly preferably a sodium platinate, particularly preferably sodium hexachloroplatinate (Na₂PtCl₆) and/or sodium hexahydroxyplatinate (Na₂Pt(OH)₆), is metered into the catholyte in the electrolysis of sodium chloride, characterized in that the addition is carried out during electrolysis operation at a current density of from 0.2 A/m² to 95 A/m², preferably from 0.5 A/m² to 70 A/m², particularly preferably from 1 A/m² to 50 A/m², at a temperature of the catholyte in the range from 40° C. to 95° C., using an amount of platinum per m² of electrode area of from 0.3 g/m² to 10 g/m², preferably from 0.35 g/m² to 8 g/m², particularly preferably from 0.4 g/m² to 5 g/m, with the decreased current density being maintained from the commencement of the metered addition for a total of from 2 to 360 minutes, preferably from 20 minutes to 300 minutes, particularly preferably from 30 minutes to 200 minutes.

For the purposes of the invention, the amount of platinum refers to the content of platinum metal in the platinum compound introduced.

Electrode area here means, in particular, the total active electrode area wetted by the catholyte. In the interests of simplicity, the electrode area preferably refers to the geometric dimensions of the active electrode area wetted by the catholyte.

In particular, the sodium hexachloroplatinate can, as desired, either be introduced as aqueous solution or in alkaline solution into the catholyte or the hexachloroplatinic acid is introduced directly into the catholyte, in particular the sodium hydroxide solution, with a reaction with the alkali to form sodium chloroplatinate then occurring.

To avoid precipitation of platinum metal particles in the catholyte at a high current density as in the processes known from the prior art, the addition of the platinum compound is, according to the invention, carried out during ongoing electrolysis under a greatly reduced load, i.e. the current density is set to not more than 95 A/m² for the introduction of platinum.

In a further preferred embodiment of the addition of platinum, the temperature of the catholyte is from 60 to 90° C., preferably from 75 to 90° C., during addition of the platinum compound.

In a preferred embodiment of the invention, the electrode coating is present in the form of platinum metals and/or platinum metal oxides on the coated nickel electrodes, where the platinum metals/platinum metal oxides are based on one or more metals of the group consisting of: ruthenium, iridium, palladium, platinum, rhodium and osmium, particularly preferably on those of the group consisting of: ruthenium, iridium and platinum.

In a further preferred embodiment of the new method, not only the abovementioned soluble platinum compound but in addition other further water-soluble compounds of the noble metals of transition group 8 of the Periodic Table of the Elements, in particular compounds of palladium, iridium, rhodium, osmium or ruthenium, preferably palladium or ruthenium, are added to the catholyte. In particular, these are used in the form of water-soluble salts or complex acids.

In a preferred new method, the proportion of noble metal of the further water-soluble compounds of the noble metals of transition group 8 is from 1 to 50% by weight, based on the platinum metal of the soluble platinum compound.

A preferred variant of the new method is characterized in that the proportion of platinum in the platinum compound in the catholyte after the addition is from 0.01 to 310 mg/l, preferably from 0.02 to 250 mg/l, particularly preferably from 0.03 to 160 mg/l.

In a preferred variant of the new method, the volume flow of the catholyte during the addition is from 0.1 to 10 l/min, preferably from 0.2 to 5 l/min

To avoid unnecessary losses of platinum metals, the concentration of platinum metal in the catholyte exiting from the electrolysis cell is continuously or discontinuously monitored in a particularly preferred embodiment of the new method.

The sodium chloride electrolysis by the membrane process is typically carried out, by way of example, as follows. A solution containing sodium chloride is fed into an anode chamber having an anode, while a sodium hydroxide solution is fed into a cathode chamber having a cathode. The two chambers are separated by an ion-exchange membrane. A plurality of these anode and cathode chambers are assembled to form an electrolyzer. Apart from the chlorine formed, a sodium chloride-containing solution having a lower concentration than that fed to the anode chamber leaves the anode chamber. Hydrogen and a sodium hydroxide solution having a higher concentration than that fed into the cathode chamber leave the cathode chamber. The production current density is, for example. 4 kA/m². The geometrically projected cathode area is 2.7 m², which corresponds to the membrane area. The cathode consists of a nickel expanded metal provided with a specific coating (here also variously described simply as coating) (manufacturer: for example Industrie De Nora) in order to decrease the hydrogen overvoltage.

The invention further provides a process for producing chlorine, sodium hydroxide and hydrogen according to the principle of membrane electrolysis on a production scale using nickel electrodes or coated nickel electrodes as cathode, comprising the steps:

-   -   introduction of an aqueous solution containing sodium chloride         into an anode chamber having an anode and introduction of sodium         hydroxide solution into a cathode chamber having a cathode,         where anode chamber and cathode chamber are separated from one         another by an ion-exchange membrane;     -   setting of a production current density of at least 1 kA/m²         based on the electrode area;     -   discharge of the solution containing sodium chloride from the         anode chamber together with the chlorine gas formed at the anode         and separation of the chlorine from the liquid phase;     -   feeding of the chlorine which has been separated off to a         suitable treatment, in particular comprising at least drying,         purification and optionally compression of the chlorine gas;     -   feeding of the sodium chloride-containing solution discharged         from the anode space to concentration and purification, where         the concentration and purification comprises, in particular, at         least the following steps: destruction of chlorate by-products,         dechlorination, increasing of the concentration by addition of         sodium chloride, purification by means of precipitation         reagents, filtration and ion exchange to remove undesirable         cations,     -   subsequent reintroduction of the solution containing sodium         chloride into the anode chamber;     -   discharge of the sodium chloride solution from the cathode         chamber together with the hydrogen formed at the cathode and         separation of the hydrogen from the liquid phase;     -   optionally feeding of the hydrogen which has been separated off         to a suitable treatment and purification;     -   feeding of the sodium hydroxide solution discharged from the         cathode chamber to a collection vessel and optionally to a         further suitable treatment and purification;     -   dilution of a partial amount of the sodium hydroxide solution         discharged from the cathode space with water and reintroduction         into the cathode space;

characterized in that the current density is reduced to a value of less than 100 A/m² but at least 0.2 A/m² in order to lower the electrolysis voltage on attainment of a prescribed average maximum voltage value during electrolysis operation, the method as claimed in any of claims 1 to 8 is then carried out and the current density is subsequently increased again to the production current density and production is continued.

Here, production current density is, in particular, a current density of at least 1 kA/m².

Here, production scale is, in particular, the conversion of at least 5 kg/h of sodium chloride into chlorine and sodium hydroxide per electrolysis cell.

In particular, the maximum voltage value is in the case of individual cells the maximum electrolysis voltage across the individual cell which is considered to be tolerable in respect of energy efficiency of the electrolysis process. This threshold value is typically about 80 mV above the best average voltage value after start-up of the cell.

In the case of electrolyzers having a plurality of individual cells, the average of the measured voltages is used as comparative value in the interests of simplicity.

In a preferred embodiment of the novel electrolysis process, the concentration of the solution containing sodium chloride is at least 150 g/l.

In a further preferred embodiment of the novel electrolysis process, the content of NaOH in the sodium hydroxide solution is at least 25% by weight.

Sodium chloride-containing solution and sodium hydroxide solution are preferably heated to at least 60° C. before introduction.

In a further preferred embodiment of the novel electrolysis process, the sodium chloride-containing solution is brought to a pH below 6.

EXAMPLES

The following experimental examples were carried out on industrial electrolyzers each having 144 elements (individual electrolysis cells) whose nickel cathodes were provided with a coating based on a mixture of ruthenium/ruthenium oxide from the Denora company.

The average voltage for each electrolyzer was calculated from the average of the 144 elements. The voltage values having a current density in electrolysis operation of 4.5 kA/m² were employed for comparison of the voltages or voltage changes in the electrolysis.

In the case of no voltage values being available at this current density because the electrolyzer was not operated at this current density at the point in time retrieved in each case, the measured voltage was converted by calculation to the voltage corresponding to the current density of 4.5 kA/m². The conversion was carried out by means of a linear regression of the current-voltage data in the range from 3 to 5 kA/m². In this current range, the current-voltage characteristics of an electrolyzer are linear.

Example 1

An industrial electrolyzer was operated at an average voltage of 3.27 V and a current density of 4.5 kA/m².

The following procedure was carried out:

The current density was decreased from 4.5 kA/m² to a current density of 11.8 A/m² over a period of 30 minutes and kept constant at this value. After 10 minutes, 8 l of a solution of hexachloroplatinate (25 g of Pt/l) were metered at 0.8 l/min into the sodium hydroxide solution (32%) over a period of 10 minutes. The proportion of platinum of the platinum compound in the sodium hydroxide solution increased to 16 mg/l here. The current density remained at the constant value of 11.8 A/m² and after the addition was complete was kept at this value for a further 30 minutes. Overall, the time for which the current density was kept at 11.8 A/m² from the commencement of the addition was 40 minutes. The current density was then increased again to 4.5 kA/m² over a period of 45 minutes.

The temperature of the sodium hydroxide solution varied in the range from 76 to 90° C. over the total procedure.

The volume flow of sodium hydroxide solution during the addition time was 3.6 l/min per element.

Thus, 200 g of platinum were brought to the surface of 144 cathodes (surface area of a cathode: 2.7 m²). This corresponds to an amount of platinum of 0.51 g/m².

The average voltage at 4.5 kA/m² dropped from the initial value of 3.27 V to 3.10 V after the addition. This corresponds to a voltage decrease of 170 mV.

After 126 days of operation, the average voltage at 4.1 kA/m² was 3.07 V. Converted to a current density of 4.5 kA/m², this corresponds to an average voltage of 3.13 V. The voltage decrease is 140 mV.

After a stoppage and a total of 129 days of operation after the metered addition, the average voltage at 4.5 kA/m² was 3.16 V. The voltage decrease is 110 mV.

After a further stoppage and a total of 133 days of operation after the metered addition, the average voltage at 4.5 kA/m² was 3.17 V. The voltage decrease is 100 mV.

Example 2 Comparative Example

An industrial electrolyzer was operated at an average voltage of 3.15 V and a current density of 4.2 kA/m². Converted to a current density of 4.5 kA/m², this corresponds to a voltage of 3.19 V.

The following procedure was carried out:

During ongoing operation, 6 l of a solution of hexachloroplatinate (7.1 g of Pt/l) was metered at 1 l/h into the sodium hydroxide solution (32%, 90° C.) over a period of 6 hours. The current density varied in the range from 4.3 to 4.7 kA/m² here.

Thus, 43 g of platinum was brought to the surface of 144 cathodes (surface area of a cathode: 2.7 m²). This corresponds to an amount of platinum of 0.11 g/m².

After the addition of the hexachloroplatinate solution was complete, an average voltage of 3.17 V was obtained at a current density of 4.7 kA/m². Conversion to a current density of 4.5 kA/m² gives an average voltage of 3.14 V. This corresponds to a voltage decrease of 50 mV.

After 5 days of operation, an average voltage of 3.16 V was measured at 4.5 kA/m². The voltage decrease is thus only 20 mV.

After a total of 8 days of operation after the metered addition, the electrolyzer was shut down. After the shutdown, an average voltage of 3.17 V was measured at 4.4 kA/m². Converted to 4.5 kA/m², this corresponds to an average voltage of 3.18 V. The voltage decrease originally achieved has thus been virtually completely eliminated.

Example 3

Further Comparative Example

An industrial electrolyzer was operated at an average voltage of 3.17 V and a current density of 4.3 kA/m². Converted to a current density of 4.5 kA/m², this corresponds to a voltage of 3.2 V.

The following procedure was carried out:

The current density was decreased from 4.3 kA/m² to a current density of 11.8 A/m² over a period of 30 minutes and kept constant at this value. After 10 minutes, 8 l of a solution of hexachloroplatinate (6.25 g of Pt/l) were metered at 0.8 l/h into the sodium hydroxide solution over a period of 10 minutes. The current density here remained at the constant value of 11.8 A/m2 and after the addition was complete was kept at this value for a further 30 minutes. Overall, the time for which the current density was kept at 11.8 A/m² from the beginning of the addition was 40 minutes. The current density was then increased to 3.8 kA/m² over a period of 45 minutes.

The temperature of the sodium hydroxide solution over the total procedure varied in the range from 76 to 90° C.

Thus, 50 g of platinum were brought to the surface of 144 cathodes (surface area of a cathode: 2.7 m²). This corresponds to an amount of platinum of 0.13 g/m².

After the addition, an average voltage of 3.0 V was determined at a current density of 3.8 kA/m². Converted to a current density of 4.5 kA/m², this corresponds to an average voltage of 3.1 V. The voltage decrease is accordingly 100 mV.

After a total of 8 days of operation after the metered addition and a shutdown, an average voltage of 3.19 V was measured at a current density of 4.5 kA/m². The voltage decrease is accordingly only 10 mV and has thus been virtually completely eliminated. 

1-9. (canceled)
 10. A method for improving the performance of nickel electrodes which are uncoated or have a coating based on platinum metals, platinum metal oxides or a mixture of platinum metals and platinum metal oxides and are used in sodium chloride electrolysis by the membrane process, where a platinum compound which is water-soluble or soluble in sodium hydroxide solution, in particular hexachloroplatinic acid or a sodium platinate, particularly preferably Na₂PtCl₆ and/or Na₂Pt(OH)₆ is metered into the catholyte during the electrolysis of sodium chloride, wherein the addition is carried out during electrolysis operation at a reduced current density of from 0.2 A/m² to 95 A/m², preferably from 0.5 A/m² to 70 A/m², particularly preferably from 1 A/m² to 50 A/m², at a temperature of the catholyte in the range from 40° C. to 95° C., using an amount of platinum per m² of electrode area of from 0.3 g/m² to 10 g/m², preferably from 0.35 g/m² to 8 g/m², particularly preferably from 0.4 g/m² to 5 g/m², with the decreased current density being maintained from the commencement of the metered addition for a total of from 2 to 360 minutes, preferably from 4 minutes to 300 minutes, particularly preferably from 5 minutes to 200 minutes.
 11. The method as claimed in claim 10, wherein not only the platinum compound but also further other water-soluble compounds of the noble metals of transition group 8 of the Periodic Table of the Elements, in particular compounds of the platinum group, particularly preferably of palladium, iridium, rhodium, osmium or ruthenium, preferably palladium or ruthenium, are added.
 12. The method as claimed in claim 11, wherein the proportion of noble metal of the further water-soluble compounds of the noble metals of transition group 8 is from 1 to 50% by weight, based on the platinum metal of the soluble platinum compound.
 13. The method as claimed in claim 10, wherein the temperature of the catholyte at which the metered addition of the platinum compound is carried out is in the range from 60 to 90° C.
 14. The method as claimed in claim 10, wherein the proportion of platinum of the platinum compound in the catholyte after the metered addition is from 0.01 to 310 mg/l.
 15. The method as claimed in claim 10, wherein the volume flow of the catholyte during the contact time of the electrode surface with the catholyte containing the platinum compound is from 0.1 to 10 l/min.
 16. The method as claimed in claim 10, wherein the concentration of platinum metal in the catholyte exiting from the electrolysis cell is continuously or discontinuously monitored.
 17. The method as claimed in claim 10, wherein the method is carried out on coated nickel electrodes, with the coating comprising platinum metal/platinum metal oxide based on one or more metals from the group consisting of: ruthenium, iridium, palladium, platinum, rhodium and osmium, preferably from the group consisting of: ruthenium, iridium and platinum.
 18. A process for producing chlorine, sodium hydroxide and hydrogen according to the principle of membrane electrolysis on a production scale using nickel electrodes or coated nickel electrodes as cathode, comprising the steps: introduction of an aqueous solution containing sodium chloride into an anode chamber having an anode and introduction of sodium hydroxide solution into a cathode chamber having a cathode, where anode chamber and cathode chamber are separated from one another by an ion-exchange membrane; setting of a production current density of at least 1 kA/m² based on the electrode area; discharge of the solution containing sodium chloride from the anode chamber together with the chlorine gas formed at the anode and separation of the chlorine from the liquid phase; feeding of the chlorine which has been separated off to a suitable treatment, in particular comprising at least drying, purification and optionally compression of the chlorine gas; feeding of the sodium chloride-containing solution discharged from the anode space to concentration and purification, in particular comprising at least the steps: destruction of chlorate by-products, dechlorination, increasing of the concentration by addition of sodium chloride, purification by means of precipitation reagents, filtration and ion exchange to remove undesirable cations, subsequent reintroduction of the solution containing sodium chloride into the anode chamber; discharge of the sodium chloride solution from the cathode chamber together with the hydrogen formed at the cathode and separation of the hydrogen from the liquid phase; optionally feeding of the hydrogen which has been separated off to a suitable treatment and purification; feeding of the sodium hydroxide solution discharged from the cathode chamber to a collection vessel and optionally to a further suitable treatment and purification; dilution of a partial amount of the sodium hydroxide solution discharged from the cathode space with water and reintroduction into the cathode space; wherein the current density is reduced to a value of less than 100 A/m² but at least 0.2 A/m² in order to lower the electrolysis voltage on attainment of a prescribed average maximum voltage value during electrolysis operation, the method as claimed in claim 10 is carried out and the current density is subsequently increased again to the production current density and production is continued. 